Other devices are known in zigzag sewing machines in which both the bight and the needle position of needle vibration can be separately adjusted. Reference is made to the U.S. Pat. No. 2,047,754 of Tiesler which discloses such a zigzag control mechanism.
By bight, is meant the width of lateral zigzag motion of the sewing machine needle between successive penetrations. By neutral position of needle vibration, is meant the point within the total possible bight of the needle movement to which the needle will revert when the bight is reduced to zero. Zigzag control mechanisms are usually built so that the neutral position of needle vibration can be set either in the extreme left-hand side of the widest possible bight of zigzag stitching or to the extreme right-hand side thereof, as well as any position intermediate these side extremes. The control for influencing neutral position of needle vibration is sometimes referred to simply as the position control and it is that control which enables the operator to sew straight stitching at either the left, center or right-hand side of the elongated needle aperture in the sewing machine throat plate.
In instances in which a sewing machine will always be fitted with a throat plate having the same size elongated needle aperture sufficient to accommodate the largest possible bight adjustment provided by the control device for the zigzag mechanism, control devices such as disclosed in the above referenced U.S. Pat. No. 2,047,754 are adequate. There are instances, however, particularly in industrial sewing machine usage in which it is required to provide a throat plate with a needle aperture elongated less than that amount which could accommodate the maximum possible bight which the zigzag control mechanism is possible of providing. One possible reason for providing a throat plate with a needle accommodating aperture having a width less than the possible maximum value would be to minimize flagging of the work which the wider needle aperture would occasion in instances in commercial usage in which only narrow zigzag stitches are required. If under these circumstances a control mechanism of the Tiesler U.S. Pat. No. 2,047,754 were to be used, a difficulty would be experienced in that the left and right hand settings of the neutral position of needle vibration control cannot be made to occur at the extreme left and right hand extremities of the needle accommodating aperture in the throat plate. This is so in known zigzag mechanisms because from any setting of the position control other than its extreme left or right settings, the zigzag width and bight will grow in both left and right hand directions as the bight control is advanced from zero bight setting. If, therefore, a device such as the Tiesler U.S. Pat. No. 2,047,754 teaches were to be used with a throat plate having a slot less than that which is able to accommodate the total maximum bight possible, skewed patterns of zigzag stitches will result in which the needle cannot be made to sew straight stitches in the extreme left and right hand positions.